1. Field of the Invention
The field of the invention is that of telecommunications. To be more precise, the present invention relates to a secure method of deciding on a given state of a communication channel in a data decompression device.
The invention also relates to a data transmission system.
2. Description of the Prior Art
FIG. 1 shows a prior art satellite data transmission network which includes a satellite 11 and a communication resource management center 10 that communicates by radio with the satellite 11. Traffic stations 12, 13 comprising terminals operating in TDMA or SCPC mode also communicate with the satellite 11 and are connected to public or private telephone switching centers 14, 15, usually referred to as a public switched telephone network (PSTN) in the case of a terrestrial network. Each PSTN 14, 15 is connected to a plurality of users 16, 17.
Calls between users 16 and users 17 connected to different traffic stations are set up by the management center 10 which dynamically allocates transmission frequencies (in SCPC operating mode) or time slots of a time frame (in TDMA operation) as a function of connection requests from these users. This is known as demand assignment multiple access (DAMA) and this dynamic allocation of resources optimizes the use of satellite resources.
Thus satellite resources are assigned on demand; when a user requests a call, and if his request can be honored, a satellite channel is set up between the outgoing traffic station to which the requesting user is connected and an incoming traffic station to which the called party is connected. The management center 10 is also informed of the releasing of assigned resources at the end of a call.
The center 10 not only manages satellite frequencies but also the making available of modems in the outgoing and incoming traffic stations for setting up the telephone connections.
Operation is generally as follows:
In the SCPC mode of operation, the management center 10 assigns satellite frequencies when it detects line seizure by a user 16 or 17, that line seizure being manifested in an analog signal (at a particular frequency) or a digital signal (line seizure signaling bit or word) transmitted by the user to the management center 10 via the PSTN 12 or 13. The traffic stations 14 and 15 shape the signals sent by the users for transmission to the management center 10 via a modem.
One such frame is shown by way of example in FIG. 2. The frame 20 comprises 32 time slots IT1 to IT32, each of eight bits, of which the first time slot IT1 is dedicated to particular signaling and synchronization, the time slot IT16 conveys line signaling from the PSTN, and the other time slots are reserved for transmitting payload data (dialing, voice data, etc.) sent by the users for one transmission direction. The users constitute telephones, private branch exchanges or a public telephone network. Each frame has a duration of 125 μs and provides a communication bit rate of 2 Mbit/s.
FIG. 3 shows diagrammatically part of the infrastructure of a Global System for Mobile communications (GSM) network. It shows the radio subsystem 21 representing the base station system (BSS) managing the radio transceiver stations. A BSS comprises a base station controller (BSC) 22 and one or more cells and thus one or more base transceiver stations (BTS) 23. The BSC manages the radio resources of the BTS attached to it and the operation and maintenance functions of the base transceiver station. It autonomously executes handover of mobile stations moving around in its coverage area. Furthermore, as shown in FIG. 3, the BSC has two standardized interfaces, a A-bis interface with the base transceiver stations 23 and an A-ter interface connecting the BSC to a mobile switching center (MSC) 24 via a transcoder rate adapter unit (TRAU) 25. The purpose of this is to convert compressed voice at 13 kbit/s to digitized speech at 64 kbit/s in order to render the speech channels compatible with the MSC. Thus the MSC-BSC coupling is effected at a standard bit rate of 64 kbit/s on the MSC side and of 16 kbit/s on the BSC side, this bit rate comprising the bit rate of the compressed voice at 13 kbit/s plus an additional bit rate consisting of framing and stuffing bits. The interface between the MSC and the TRAU is called the A interface; the interface between the TRAU and the BSC is called the A-ter interface.
The TRAU 25 is compatible with the various signal types transmitted at the A-ter interface, and converts all these signal types to a bit rate of 64 kbit/s. These signals are essentially voice at 16 kbit/s (full rate) or 8 kbit/s (half rate) and signaling at 64 kbit/s or 16 kbit/s.
The MSC is the interface between the BSS and a cable network such as a public land mobile network (PLMN) 27. The MSC carries out all operations needed for managing calls involving mobile terminals. To obtain radio coverage of a territory, a mobile network switch controls a set of senders, which explains the presence in FIG. 3 of a plurality of A-ter interfaces with other BSS.
The A-bis interface providing the connection between the BTS and the BSC of the system is established via a synchronous interface E1 using G.703 frames (referred to as E1 frames). A portion of each frame carries payload data.
It will be noted that of a GSM network expansion via satellite, in particular as proposed hereinafter, is effected either at the A-bis interface or at the A-ter interface, or possibly at the A interface.
Regardless of which interface is selected for network expansion via satellite, the number of transmission channels used (time slots or subdivisions of time slots) is fixed and depend essentially on the physical configuration of the BSS (number of BTS, number of carriers). On the other hand, at a given time, only some of the transmission channels are active; the number of active channels depends on the signaling to be carried, the number of calls that have been set up, and on the inherent half duplex nature of dialog between parties.
To minimize the bandwidth required for the satellite communication for network expansion via satellite, the telecommunication system considered for the network expansion functions in DAMA mode, i.e. the satellite resources dedicated to the connection at a given time depend on the bit rate of the data to be transmitted, i.e. on the number of active channels within the frames to be transmitted.
The equipment enabling the DAMA technique to be used operates in two different modes:                either the equipment interprets signaling (for example SS7 signaling) to detect the activation of new transmission channels in order to adapt the assignment of transmission resources accordingly (variation of the band assigned for a given connection); in this case, the signaling is not standard signaling, and having the DAMA operate as a function of the signaling carried would be complicated and would depend on the equipment supplier, the A-bis interface between the BSC and the BTS not being standardized,        or the equipment is at the Ethernet, ATM, or even Frame Relay interface; in this case, the DAMA process functions more simply, because it takes the average bit rate on the transmission channel as its base for adjusting transmission resource assignment. Note that in the present case the bit rate is invariant, because it is independent of the activity of the channels to be transmitted via the satellite, and is typically equal to 8×16 kbit/s per carrier transmitted by the BTS.        
This second version of DAMA, based on measuring or detecting bit rate variations, is preferred because it avoids having to interpret the signaling carried at the remote interface in order to vary the satellite band assignment. However, because the network expansion interface is not directly compatible with the transmission equipment of the system, an intermediate device known as a transcoder is used.
A two-fold requirement is imposed for the transcoder: firstly, it must be able to extract from synchronous frames the payload data corresponding to active transmission channels, and only those channels, and then encapsulate them in Ethernet frames, IP packets or ATM cells. These elements are fed to the transmission equipment of the BSC, which can therefore offer the benefit of DAMA.
Moreover, the transcoder must also be able to restore synchronism at the end of the transmission system, in that the process introduced, based on extracting payload data from synchronous frames and encapsulation, completely breaks the original frame sequence. Consequently, the transcoder must be able to reconstitute the frames identically as they existed at the source.
To obtain the benefit of the DAMA functions offered by the transmission equipment, the E1 frames must be converted into Ethernet frames, IP packets, or an ATM stream. Prior art transcoders can handle E1-Ethernet or E1-IP adaptation, or E1-ATM adaptation if there is no compression of the frames E1 to be transmitted. Whether the frames E1 carry valid data or not, the resultant bit rate is constant; thus these transcoders cannot reduce the satellite bandwidth as a function of the effective activity of the GSM network transmission channels; the reason these transcoders ignore the real activity of the transmitted channels is associated with the fact that they constitute interface conversion solutions and do not analyze the content of the frame.
To eliminate the drawbacks cited above, the Applicant has this day filed a French patent application (referred to hereinafter as the INV1 application) with the French title “Compresseur, décompresseur, bloc de données et procédé de gestion de ressources” [translated as “Compressor, decompressor, data block and resource management method”]. The INV1 application describes a compressor for compressing data from input data frames. According to the invention, the compressor detects the bits (carrying a communication channel) that vary from one frame to another (are “active”) and transmits only these in the data block. The block also contains a state code locating the “active” data in the original frame in order to be able to reconstitute it.
The compression process takes place within an analysis window that contains a fixed number of input frames (for example 16, 32, 64 or 128 input frames).
A channel is treated as non-active, or static, as soon as the same pattern, as regards its content, recurs periodically during the analysis window. For example, this kind of situation arises when, for a given channel, i.e. for a given bit, the pattern 1110 is reproduced by input frame groups in input frames. This pattern is identified as a reference pattern for the bit considered. If the analysis window contains 16 frames (has a length of 16 frames), for example, the pattern 1110 occurs four times in succession in the analysis window (i.e. there are four groups).
At the other end of the data compression system, each compressed data block is decompressed using an active channel descriptor inserted into the header of the block in order to restore the original structure of the input frames, by replacing each channel at the place it occupied before compression. In this way, N output frames identical to the input frames are reconstituted from a compressed data block; for the non-active channels, the decompression device functions by repeating the reference state all along the current window. The reference state for each static channel is obtained from a reference pattern that is transmitted periodically, for example between two data blocks.
However, despite the ingenuity of the principle explained above, it can suffer seriously from errors in transmitting bits included in the static bit groups of the analysis window.
Returning to the above example, if the analysis window contains the following sequence of bits:                1110111011101110for a given channel and over 16 frames, then, the content of that channel reproducing the content of the reference state for this bit, which is 1110, during 16 frames, the bit will be considered static and the decompression device will act accordingly.        
However, following a transmission error, if at least one of the 16 bits is transmitted incorrectly, the reference state of the channel in question is no longer identically reproduced and the decompression device will analyze it as an active bit, although this is not the case in reality.
Thus the invention aims to solve this major problem.